Push-to-talk (PTT) service over cellular or wireless networks provides a type of half-duplex or one-way communications service between two or more users. See e.g., the Push-to-talk over Cellular (PoC) architecture, of the Open Mobile Alliance Ltd. Users often form a group, a predefined set of users, who then communicate with each other in a “point-to-multipoint” fashion. The communications are one-way: while one person (user) speaks, the other(s) listen(s). Turns to speak are generally granted on a first come, first serve basis by a user pressing a push-to-talk key or button on his/her wireless phone (or other user equipment). However, the PTT service provider may apply some type of arbitration process in order to insure “fair” access among the members of a PTT session.
Push-to-talk functionality is typically delivered across a mobile operator's private Internet Protocol (IP) data network using standard VoIP (Voice over IP) protocols, although other technology implementations are possible. In a typical IP network scenario, a mobile phone user with a push-to-talk enabled phone enters a PTT address for another mobile user who is registered for push-to-talk services and then presses the push-to-talk key on his/her mobile phone to initiate a session. In a typical scenario, the address of the destination mobile user can be based either on a mobile phone number (e.g., 1-978-555-1212) or on an Internet address in the form of a network access identifier (e.g., user@operator.com). If the destination user is available, the originator receives an immediate indication that the destination user is available and the originator can begin speaking. The call originator's voice is sent through the wireless operator's private IP network to the target handset. The call connection over the operator's IP network is almost instantaneous. Initiating a push-to-talk session to a mobile/wireless user who is not available simply results in a negative response tone.
FIG. 1 illustrates an example of a wireless operator's push-to-talk service (PTT service) as described in U.S. Pat. No. 6,360,093 to Ross. A wireless phone (102) with a push-to-talk button (104) registers with a push-to-talk server (PTT server) (110) located on the wireless operator's IP network (108), using standard VoIP protocols. A base station (106) provides the interface between the radio signals used to communicate with the wireless phone and the service provider's IP network. The PTT server (110) maintains a database of registered wireless phones (102, 103, 105) and associated Internet addresses. The PTT server acts as the clearance or acceptance point for all push-to-talk sessions with its registered wireless users. All push-to-talk session requests are first presented to the PTT server which then forwards or proxies the request (via base stations 106, 107) to the appropriate registered destination wireless phone(s). A network computer (112) with PTT functionality (109) may also be configured to emulate a combined base station and wireless phone and register for push-to-talk services with the wireless operator's PTT server.
A PTT server can establish a push-to-talk session between two or more of its locally registered wireless phones or network computers. The PTT server(s) of one PTT service provider can also be configured to communicate with a known set of PTT servers of one or more other PTT service providers to enable push-to-talk sessions between two or more wireless phones or computers, each of which is registered with a different PTT service provider. What it cannot do, however, is establish a push-to-talk session with a push-to-talk enabled computer that is not registered with any operator's PTT service.